The present invention relates to a protective circuit for a battery, and more particularly a protective circuit for a battery adapted to protect a battery against too high charge or discharge currents.
In the rechargeable battery field, protective circuits for batteries are known by means of which battery overcharge, which is undesirable for safety reasons, can be avoided. Protective circuits for batteries by means of which over-discharging of the battery can also be avoided are also known. Indeed, particularly in the case of Li-ion (Lithium-ion) batteries, it may be desirable to avoid such over-discharging of the battery which would have the effect of shortening its lifetime. Typically, such circuits are adapted to measure the voltage across the terminals of the battery and interrupt the charging or discharging thereof in response to a signal indicating that the voltage is greater than or less than a predetermined limit.
In addition to the protective functions against overcharging or over-discharging, protective battery circuits are typically also adapted to protect the battery, during normal operation, i.e. when the battery is neither overcharged nor excessively discharged, against too high charge or discharge currents. One then speaks commonly of overcurrents.
A protective battery circuit of this type typically includes current interruption means connected in series with the battery so that when a load or a charger are connected to the terminals of the battery and the interruption means, a discharge or respectively a charge current, of the opposite direction, flows through the battery and the current interruption means. The current interruption means consequently have to be adapted to conduct and, if necessary, interrupt the current in a bi-directional manner, i.e. both during charging and discharging of the battery.
The making of the current interruption means typically involves using MOS transistors whose conduction can easily be controlled by means of their gate. In particular, a first solution consists in using two power MOSFET transistors connected in anti-series, i.e. source to source or alternatively, drain to drain. Thus, when an overcharge condition is detected, one of the transistors is switched off in order to interrupt the flow of the charge current. Similarly, when an over-discharge condition is detected, the other transistor is switched off in order to interrupt the flow of the discharge current. It will be noted that when one or other of the transistors is switched off, the flow of a current in the opposite direction is nonetheless allowed, since all power transistors have a parasitic diode formed, in parallel with the channel, between the drain and the source, so that when an overcharge condition is detected for example, a charge current can still flow through the transistors. In normal operation, i.e. in the absence of overcharging or over-discharging, the two transistors are conductive.
Alternatively, using a bi-directional switch has also been proposed, formed of a single MOSFET transistor to perform this function. Examples of such a bi-directional switch are presented in U.S. Pat. Nos. 5,689,209 and 5,581,170. This solution is particularly advantageous since the transistor conduction resistance is divided in two compared to the solution using two power MOFSET transistors connected in anti-series. Moreover, the surface necessary to integrate such a bi-directional transistor is also less and consequently allows costs to be reduced.
In order to detect an overcharge, measuring a voltage drop across the terminals of measuring resistor, arranged in series with the battery in the path of the charge or discharge current, and interrupting the flow of the current when this voltage drop exceeds a predetermined value is for example proposed. This solution is however not very suitable, since it is generally desirable to limit the resistance present in series with the battery in the path of the current. Moreover, it is relatively difficult to accurately control the value of this resistance when it is made in an integrated way. This lack of precision then affects the value of the measured charge or discharge current.
A proposed alternative consists in measuring the voltage across the terminals of the current interruption means, this voltage being representative of the charge or discharge current which passes through them. When the voltage exceeds a determined reference voltage corresponding to a limit current value, the current interruption means are then activated in order to prevent the current flowing through the battery.
Since the current interruption means are typically made, as mentioned, by means of a pair of power MOFSET transistors mounted in anti-series or alternatively by means of a single MOFSET transistor, the voltage measured across their terminals is thus representative of the drain-source voltage VDS of the MOFSET transistor(s). For a given drain current ID, i.e. for a given charge or discharge current, this voltage VDS is dependent in particular on the gate voltage VGS of the transistor. This voltage is also dependent on the transistor""s own characteristics, in particular the threshold voltage and the gate oxide capacitance of the transistor. The voltage is also substantially dependent on the temperature.
FIG. 1 shows a diagram of the output characteristics ID/VS of an MOS transistor in its linear zone, i.e. for low drain-source voltage values VDS less than the transistor saturation voltage and for a given gate voltage value VGS. Two curves a and b are illustrated in this Figure. Curve a illustrates the linear zone output characteristic of the transistor for a nominal temperature T0. Following an increase in temperature to a higher temperature T1, the transistor output characteristic, for the same gate voltage VGS, as illustrated by curve b, tends to become lower. It will thus be noted that for a drain-source voltage value corresponding to given a drain current I0, at nominal temperature T0, this voltage is reached for a drain current I1 less than current I0 when the temperature increases to T1. In other words, the measured voltage for a given current, for example equal to a current threshold IPROT, increases with the temperature. Thus, if one chooses, as is typically the case, to fix a reference voltage VREF representative of this current threshold IPROT and to compare the measured voltage to this reference voltage, the real current value will differ according to the temperature.
Moreover, the reference voltage VREF may also vary substantially with the temperature. In particular reference voltage VREF can typically be more strongly temperature dependent than the voltage measured across the terminals of the interruption means. In such case, the effective current can exceed the current threshold defined by the reference voltage before the interruption means are activated to interrupt the current flow. Conversely, if the voltage measured across the terminals of the interruption means is more strongly temperature dependent than reference voltage VREF, the interruption means are liable to interrupt the current flow while the latter is substantially less than the fixed current threshold.
An object of the present invention is thus to overcome these drawbacks and to provide a protective battery circuit wherein the temperature dependence of the measured voltage and/or the reference voltage, can be compensated.
The present invention concerns a protective battery circuit the features of which are defined in the appended claims.
As a result of these features, it is possible, in particular, to ensure that the charge or discharge current flowing through the battery does not exceed a determined current threshold, whatever the temperature.